1. Field of the Invention
The present invention relates to a variable-focus lens system in which a focal length is varied, and in particular, relates to a variable-focus lens system which is usable in wavelength ranges, particularly in a visible light wavelength range (about 400xcx9c700 nm) through to a near-infrared light wavelength range (about 700xcx9c1000 nm).
2. Description of the Prior Art
A surveillance camera preferably has a photographing lens system which can perform photographing operation in a visible light wavelength range at daytime, and can perform photographing operation in a near-infrared light wavelength range at nighttime. Some of such photographing lens systems have been put into practical use. However, it is still difficult to suitably correct aberrations, particularly chromatic aberration, in a visible wavelength light range over to a near-infrared light wavelength range, without making the lens arrangement complicated.
It is an object of the present invention to provide a variable-focus lens system which can suitably correct aberrations in a visible light wavelength range and a near-infrared light wavelength range.
In order to achieve the above-mentioned object, there is provided a variable-focus lens system including a negative powered (hereinafter, negative) front lens group, and a positive powered (hereinafter, positive) rear lens group, in this order from the object. The focal length of the variable-focus lens system is varied by changing the distance between the front lens group and the rear lens group, and the lens system satisfies the following conditions:
xe2x88x923.0 less than fx/Fw less than xe2x88x922.5xe2x80x83xe2x80x83(1)
3.0 less than fy/Fw less than 3.5xe2x80x83xe2x80x83(2)
wherein
fx designates the focal length of the front lens group (fx less than 0);
fy designates the focal length of the rear lens group (fy greater than 0); and
Fw designates the focal length of the entire variable-focus lens system at the short focal length extremity.
It is preferable that the rear lens group is used within a range to satisfy the following condition:
xe2x88x920.9 less than m less than xe2x88x920.3xe2x80x83xe2x80x83(3)
wherein
m designates the image-forming magnification of the rear lens group.
More concretely, the front lens group 10 can include two sub-lens groups constituted by three lens elements L1 through L3, and the rear lens group 20 includes five sub-lens groups constituted by six lens elements. Furthermore, the rear lens group 20 is preferably constituted by a positive lens element L4, a positive lens element L5, a negative lens element L6, a positive lens element L7, and positive cemented lens elements constituted by a negative lens element L8 and a positive lens element L9, in this order from the front lens group 10. In addition, the positive lens element L7 can satisfy the following conditions:
1.73 less than n7 less than 1.83xe2x80x83xe2x80x83(4)
30 less than xcexd7 less than 40xe2x80x83xe2x80x83(5)
wherein
n7 designates the refractive index of the lens element L7; and
xcexd7 designates the Abbe number of the lens element L7.
Still further, one of the lens elements L8 and L9 which constitute the cemented lens elements can be composed of a convex lens element satisfying conditions (6) and (7), and the other of the lens elements L8 and L9 can be composed of a concave lens element satisfying 13 conditions (8) and (9):
70 less than xcexd-convexxe2x80x83xe2x80x83(6)
14.0 less than f-convex/Fw less than 25.0xe2x80x83xe2x80x83(7)
30  greater than xcexd-concavexe2x80x83xe2x80x83(8)
xe2x88x927.3 less than f-concave/Fw less than xe2x88x924.5xe2x80x83xe2x80x83(9)
wherein
xcexd-convex designates the Abbe number of the convex lens element;
f-convex designates the focal length of the convex lens elements;
xcexd-concave designates the Abbe number of the concave lens element; and
f-concave designates the focal length of the concave lens elements.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-3691 (filed on Jan. 11, 2001) which is expressly incorporated herein in its entirety.